Lithium secondary battery including non-aqueous electrolyte solution

ABSTRACT

The present invention relates to a lithium secondary battery which includes a non-aqueous electrolyte solution including lithium bis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound, a positive electrode including a lithium-nickel-manganese-cobalt-based oxide as a positive electrode active material, a negative electrode, and a separator. 
     In the lithium secondary battery including a non-aqueous electrolyte solution for a lithium secondary battery of the present invention, since the non-aqueous electrolyte solution may form a robust solid electrolyte interface (SEI) on the negative electrode during initial charge and may prevent decomposition of the surface of the positive electrode and an oxidation reaction of the electrolyte solution during a high-temperature cycle, excellent low-temperature output characteristics as well as improved high-temperature storage characteristics and life characteristics may be achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No.10-2015-0138029, filed on Sep. 30, 2015, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein in itsentirety by reference.

TECHNICAL FIELD Technical Field

The present invention relates to a lithium secondary battery whichincludes a non-aqueous electrolyte solution including lithiumbis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound, apositive electrode including a lithium-nickel-manganese-cobalt-basedoxide as a positive electrode active material, a negative electrode, anda separator.

Background Art

Demand for secondary batteries as an energy source has beensignificantly increased as technology development and demand withrespect to mobile devices have increased. Among these secondarybatteries, lithium secondary batteries having high energy density andhigh voltage have been commercialized and widely used.

A lithium metal oxide is used as a positive electrode active material ofa lithium secondary battery, and a lithium metal, a lithium alloy,crystalline or amorphous carbon, or a carbon composite is used as anegative electrode active material. A current collector may be coatedwith the active material of appropriate thickness and length or theactive material itself may be coated in the form of a film, and theresultant product is then wound or stacked with an insulating separatorto prepare an electrode assembly. Thereafter, the electrode assembly isput into a can or a container similar thereto, and a secondary batteryis then prepared by injecting an electrolyte solution.

Charge and discharge of the lithium secondary battery is performed whilea process of intercalating and deintercalating lithium ions from alithium metal oxide positive electrode into and out of a graphitenegative electrode is repeated. In this case, since lithium is highlyreactive, lithium reacts with the carbon electrode to form Li₂CO₃, LiO,or LiOH, and thus, a film may be formed on the surface of the negativeelectrode. The film is denoted as “solid electrolyte interface (SEI)”,wherein the SEI formed at an initial stage of charging may prevent areaction of the lithium ions with the carbon negative electrode or othermaterials during charge and discharge. Also, the SEI may only pass thelithium ions by acting as an ion tunnel. The ion tunnel may prevent thecollapse of a structure of the carbon negative electrode due to theco-intercalation of the carbon negative electrode and organic solventsof an electrolyte solution having a high molecular weight which solvateslithium ions and moves therewith.

Therefore, in order to improve high-temperature cycle characteristicsand low-temperature output of the lithium secondary battery, a robustSEI must be formed on the negative electrode of the lithium secondarybattery. When the SEI is once formed during the first charge, the SEImay prevent the reaction of the lithium ions with the negative electrodeor other materials during repeated charge and discharge cycles caused bythe subsequent use of the battery, and the SEI may act as an ion tunnelthat only passes the lithium ions between the electrolyte solution andthe negative electrode.

Conventionally, with respect to an electrolyte solution which does notinclude an electrolyte solution additive or includes an electrolytesolution additive having poor characteristics, it may he difficult toexpect the improvement of low-temperature output characteristics due tothe formation of a non-uniform SEI. Furthermore, even in a case in whichthe electrolyte solution additive is included, if the amount of theadded electrolyte solution additive is not adjusted to a requiredamount, the surface of a positive electrode may be decomposed or anoxidation reaction of the electrolyte solution may occur during ahigh-temperature reaction due to the electrolyte solution additive, andeventually, irreversible capacity of the secondary battery may beincreased and output characteristics may be reduced. Also, since adecomposition reaction of the electrolyte solution occurs when thelithium secondary battery is stored at high temperature,high-temperature storage performance and life performance of the batterymay be degraded.

DISCLOSURE OF THE INVENTION Technical Problem

The present invention provides a non-aqueous electrolyte solution for alithium secondary battery which may improve room-temperature andhigh-temperature cycle characteristics and capacity characteristicsafter high-temperature storage as well as low-temperature androom-temperature output characteristics, and a lithium secondary batteryincluding the same.

Technical Solution

According to an aspect of the present invention, there is provided alithium secondary battery including: a non-aqueous electrolyte solutionincluding lithium bis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenylcompound, a positive electrode including alithium-nickel-manganese-cobalt-based oxide as a positive electrodeactive material, a negative electrode, and a separator.

Advantageous Effects

In a lithium secondary battery including a non-aqueous electrolytesolution for a lithium secondary battery of the present invention, sincethe non-aqueous electrolyte solution may form a robust solid electrolyteinterface (SEI) on a negative electrode during initial charge and mayprevent decomposition of the surface of a positive electrode and anoxidation reaction of the electrolyte solution during a high-temperaturecycle, excellent low-temperature output characteristics as well asimproved high-temperature storage characteristics and lifecharacteristics may be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the results of overcharge tests on batteries ofExample 1 and Comparative Example 5.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail toallow for a clearer understanding of the present invention. It will beunderstood that words or terms used in the specification and claimsshall not be interpreted as the meaning defined in commonly useddictionaries. It will be further understood that the words or termsshould be interpreted as having a meaning that is consistent with theirmeaning in the context of the relevant art and the technical idea of theinvention, based on the principle that an inventor may properly definethe meaning of the words or terms to best explain the invention.

A lithium secondary battery of the present invention includes anon-aqueous electrolyte solution including lithiumbis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound, apositive electrode including a lithium-nickel-manganese-cobalt-basedoxide as a positive electrode active material, a negative electrode, anda separator.

The non-aqueous electrolyte solution includes lithiumbis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound, and,since the lithium bis(fluorosulfonyl)imide is added as a lithium salt tothe non-aqueous electrolyte solution to form a robust thin solidelectrolyte interface (SEI) on the negative electrode, the lithiumbis(fluorosulfonyl)imide may not only improve low-temperature outputcharacteristics, but also may inhibit decomposition of the surface ofthe positive electrode, which may occur during a high-temperature cycle,and may prevent an oxidation reaction of the electrolyte solution. Sincethe fluorobiphenyl compound is added to the electrolyte solution anddecomposed in the positive electrode and the negative electrode of thelithium secondary battery including the fluorobiphenyl compound to forma thin film and the thin film plays a role in protecting the positiveelectrode to reduce metal dissolution of the positive electrode activematerial and increase porosity of a negative electrode film, lithiumions may more smoothly move, and thus, long life and storagecharacteristics of the secondary battery including the fluorobiphenylcompound may be improved. Also, the fluorobiphenyl compound may improveroom-temperature capacity characteristics and output characteristics,and, since the fluorobiphenyl compound may form a film near 4.62 Vduring overcharging to short the battery at a low state of charge (Sac),the fluorobiphenyl compound may prevent heat generation and subsequentignition of the battery. Since the SEI formed on the negative electrodeis thin, the lithium ions in the negative electrode may more smoothlymove, and, accordingly, output of the secondary battery may be improved.

According to an embodiment of the present invention, a concentration ofthe lithium bis(fluorosulfonyl)imide in the non-aqueous electrolytesolution may be in a range of 0.01 mol/L to 2 mol/L, particularly, 0.01mol/L to 1 mol/L. In a case in which the concentration of the lithiumbis(fluorosulfonyl)imide is less than 0.01 mol/L, effects of improvingthe low-temperature output and high-temperature cycle characteristics ofthe lithium secondary battery may be insignificant. In a case in whichthe concentration of the lithium bis(fluorosulfonyl)imide is greaterthan 2 mol/L, since side reactions in the electrolyte solution mayexcessively occur during charge and discharge of the battery, a swellingphenomenon may occur and corrosion of a positive electrode or negativeelectrode collector formed of a metal may occur in the electrolytesolution.

In order to prevent the side reactions, the non-aqueous electrolytesolution of the present invention may further include a lithium saltexcluding the lithium bis(fluorosulfonyl)imide. Any lithium saltcommonly used in the art may be used as the lithium salt, and, forexample, the lithium salt may include any one selected from the groupconsisting of LiPF₆, LiAsF₆, LiCF₃SO₃, LiBF₆, LiSbF₆, LiN(C₂F₅SO₂)₂,LiAlO₄, LiAlCl₄, LiSO₃CF₃ and LiClO₄, or a mixture of two or morethereof.

A mixing ratio of the lithium salt and the lithiumbis(fluorosulfonyl)imide may be in a range of 1:0.01 to 1:1 as a molarratio. In a case in which the mixing ratio of the lithium salt and thelithium bis(fluorosulfonyl)imide is above the molar ratio range, sincethe side reactions in the electrolyte solution may excessively occurduring the charge and discharge of the battery, the swelling phenomenonmay occur, and, in a case in which the mixing ratio is below the molarratio range, the output of the secondary battery generated may bereduced. Specifically, in a case in which the mixing ratio of thelithium salt and the lithium bis(fluorosulfonyl)imide is less than1:0.01 as a molar ratio, a large amount of irreversible reaction mayoccur during a process of forming the SEI in the lithium-ion battery anda process of intercalating lithium ions, which are solvated by acarbonate-based solvent, into the negative electrode, and the effects ofimproving the low-temperature output as well as the cyclecharacteristics and capacity characteristics after high-temperaturestorage of the secondary battery may be insignificant due to theexfoliation of a negative electrode surface layer (e.g., carbon surfacelayer) and the decomposition of the electrolyte solution. In a case inwhich the mixing ratio of the lithium salt and the lithiumbis(fluorosulfonyl)imide is greater than 1:1 as a molar ratio, since anexcessive amount of the lithium bis(fluorosulfonyl)imide is included inthe electrolyte solution to cause the corrosion of the electrodecollector during the charge and discharge, stability of the secondarybattery may be affected.

The positive electrode active material, as thelithium-nickel-manganese-cobalt-based oxide, may include an oxiderepresented by Formula 1 below.

Li_(1+x)(Ni_(a)Co_(b)Mn_(c))O₂   [Formula 1]

where, 0.55≤a≤0.65, 0.18≤b≤0.22, 0.18≤c≤0.22, −0.2≤x≤0.2, and x+a+b+c=1.

Since the positive electrode active material, as thelithium-nickel-manganese-cobalt-based oxide, is used in the positiveelectrode, the positive electrode active material may be combined withthe lithium bis(fluorosulfonyl)imide to have a synergistic effect. Withrespect to the lithium-nickel-manganese-cobalt-based oxide positiveelectrode active material, since a phenomenon (cation mixing), in whicha position of Li⁺¹ ion and a position of Ni⁺² ion in a layered structureof the positive electrode active material are changed during the chargeand discharge as an amount of nickel (Ni) among transition metals isincreased, occurs, the structure is collapsed, and, thus, the positiveelectrode active material may cause a side reaction with the electrolytesolution or a dissolution phenomenon of the transition metal may occur.The reason for this is that sizes of the Li⁺¹ ion and the Ni⁺² ion aresimilar. Eventually, performance of the battery is easily degraded dueto the depletion of the electrolyte solution in the secondary batteryand the structural collapse of the positive electrode active materialcaused by the side reaction.

Therefore, since he LiFSI-containing electrolyte solution is used in thepositive electrode active material of Formula 1 according to anembodiment of the present invention, a layer is formed of a componentfrom the LiFSI on the surface of the positive electrode, and thus, arange, in which a sufficient amount of the Ni transition metal forsecuring capacity of the positive electrode active material may besecured while suppressing the cation mixing phenomenon of the Li⁺¹ ionand Ni⁺ion, has been found. According to the positive electrode activematerial including the oxide according to Formula 1 of the presentinvention, the side reaction with the electrolyte solution and the metaldissolution phenomenon may be effectively suppressed when theLiFSI-containing electrolyte solution is used.

In particular, in a case in which a ratio of the Ni transition metal inthe oxide represented by Formula 1 is greater than 0.65, since anexcessive amount of the Ni is included in the positive electrode activematerial, the cation mixing phenomenon of the Li⁺¹ ion and Ni⁺² ion maynot be suppressed even by the above-described layer generated from theLiFSI on the surface of the electrode.

Also, when the excessive amount of the Ni is included in the positiveelectrode active material, the nickel transition metal having a dorbital in an environment, such as high temperature, depending on thevariation of oxidation number of the Ni must have an octahedralstructure when coordination bonded, but the order of energy levels isreversed or the oxidation number is changed (heterogenization reaction)by external energy supply to form a distorted octahedron. As a result,since a crystal structure of the positive electrode active materialincluding the nickel transition metal is transformed, the probability ofdissolution of the nickel metal in the positive electrode activematerial is increased.

As a result, the present inventors found that excellent efficiency inhigh-temperature stability and capacity characteristics is achievedwhile generating high output when the positive electrode active materialincluding the oxide in the range according to Formula 1 and the LiFSIsalt are combined.

Although the high output and stability may be achieved when the positiveelectrode active material including the oxide of Formula 1 and the LiFSIlithium salt are combined, the electrolyte solution may be decomposed ina high output environment and collapse of the negative electrode may beinduced. Thus, in a case in which the additives are combined andincluded in the non-aqueous electrolyte solution, high-temperaturestability of the secondary battery generated may be secured.

In a case in which the lithium salt is LiPF₆, the electrolyte solutionhaving insufficient thermal stability may be easily decomposed in thebattery to form LIF and PF₅. In this case, the LiF salt may reduceconductivity and the number of free Li⁺ ions to increase the resistanceof the battery, and, as a result, the capacity of the battery isreduced.

The non-aqueous electrolyte solution includes a non-aqueous organicsolvent, and the non-aqueous organic solvent is not limited as long asit may minimize the decomposition due to the oxidation reaction duringthe charge and discharge of the battery and may exhibit desiredcharacteristics with the additive. For example, the non-aqueous organicsolvent may include a nitrile-based solvent, cyclic carbonate, linearcarbonate, ester, ether, or ketone. These materials may be used alone orin combination of two or more thereof.

Among the above organic solvents, carbonate-based organic solvents maybe easily used. Examples of the cyclic carbonate may be any one selectedfrom the group consisting of ethylene carbonate (EC), propylenecarbonate (PC), and butylene carbonate (BC), or a mixture of two or morethereof, and examples of the linear carbonate may be any one selectedfrom the group consisting of dimethyl carbonate (DMC), diethyl carbonate(DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC),methylpropyl carbonate (MPC), and ethylpropyl carbonate (EPC), or amixture of two or more thereof.

The nitrile-based solvent may include at least one selected from thegroup consisting of acetonitrile, propionitrile, butyronitrile,valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile,4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile,phenylacetonitrile, 2-f luorophenylacetonitrile, and4-fluorophenylacetonitrile, and acetonitrile-based solvent may be usedas the non-aqueous solvent according to an embodiment of the presentinvention.

Side effects due to the reduction of the stability of the high-outputbattery caused by the combination with the lithiumbis(fluorosulfonyl)imide may be effectively prevented by using theacetonitrile-based solvent and using thelithium-nickel-manganese-cobalt-based oxide positive electrode activematerial in the positive electrode.

In an example of the present invention, the fluorobiphenyl compound maybe a compound represented by the following Formula 2.

In Formula 2, n may be an integer of 1 to 5, and may specifically be 2.

In an example of the present invention, the fluorobiphenyl compound maybe 2,3-difluorobiphenyl.

Since the non-aqueous electrolyte solution included in the lithiumsecondary battery of the present invention includes the fluorobiphenylcompound, the non-aqueous electrolyte solution may improve theroom-temperature capacity characteristics and output characteristics andmay prevent the heat generation and subsequent ignition of the batteryby shorting the battery at a low SOC by forming a film near 4.62 Vduring overcharging.

An amount of the fluorobiphenyl compound may be in a range of 0.5 wt %to 10 wt %, particularly 1 wt % to 7 wt %, and more particularly 3 wt %to 5 wt %, based on a total weight of the non-aqueous electrolytesolution.

In a case in which the amount of the fluorobiphenyl compound is 0.5 wt %or more, an effect of shorting the battery during the overcharging ofthe battery as well as an appropriate effect of improvingroom-temperature capacity characteristics and output characteristics maybe obtained, and, in a case in which the amount of the fluorobiphenylcompound is 10 wt % or less, problems, for example, an increase inirreversible capacity of the battery or an increase in resistance of thenegative electrode, may be prevented while having a moderate effect.

The amount of the fluorobiphenyl compound may be adjusted according tothe amount of the lithium bis(fluorosulfonyl)imide added, and,accordingly, the lithium bis(fluorosulfonyl)imide and the fluorobiphenylcompound may be used in a weight ratio of 1:0.02 to 1:10, particularly1:0.03 to 1:9, and more particularly 1:0.05 to 1:7.5.

In a case in which the lithium bis(fluorosulfonyl)imide and thefluorobiphenyl compound is used in a weight ratio of 1:0.02 to 1:10, thefluorobiphenyl compound may appropriately suppress the side reaction inthe electrolyte solution during the charge and discharge of the lithiumsecondary battery at room temperature which may occur due to theaddition of the lithium bis(fluorosulfonyl)imide, and may solve aperformance imbalance problem, for example, the reduction of the outputin comparison to capacity retention after high-temperature storage orthe reduction of the capacity retention in comparison to the output, ora problem such as a decrease in life characteristics improvement effect,when the mixing ratio is outside the above range.

The lithium secondary battery according to an embodiment of the presentinvention may include a negative electrode, a separator disposed betweenthe positive electrode and the negative electrode, and the non-aqueouselectrolyte solution. The positive electrode and the negative electrodemay respectively include the positive electrode active materialaccording to the embodiment of the present invention and a negativeelectrode active material.

Meanwhile, the negative electrode active material may include amorphouscarbon or crystalline carbon, and, for example, carbon such as hardcarbon and graphite-based carbon; a complex metal oxide such asLi_(x)Fe₂O₃ (0≤x≤1), Li_(x)WO₂ (0≤x≤1), Sn_(x)Me_(1-x)Me′_(y)O_(z)(Me=manganese (Mn), iron (Fe), lead (Pb), and germanium (Ge);Me′=aluminum (Al), boron (B), phosphorus (P), silicon (Si), Groups I, IIand III elements, or halogen; 0<x≤1; 1≤y≤3; 1≤z≤8); a lithium metal; alithium alloy; a silicon-based alloy; a tin-based alloy; an oxide suchas SnO, SnO₂, PbO, PbO₂, Pb₂O₃, Pb₃O₄, Sb₂O₃, Sb₂O₄, Sb₂O₅, GeO, GeO₂,Bi₂O₃, Bi₂O₄, and Bi₂O₅; a conductive polymer such as polyacetylene; ora Li—Co—Ni-based material may be used.

Also, a porous polymer film, for example, a porous polymer film preparedfrom a polyolefin-based polymer, such as an ethylene homopolymer, apropylene homopolymer, an ethylene/butene copolymer, an ethylene/hexenecopolymer, and an ethylene/methacrylate copolymer, may be used alone orin a lamination of two or more thereof as the separator. In addition, atypical porous nonwoven fabric, for example, a nonwoven fabric formed ofhigh melting point glass fibers or polyethylene terephthalate fibers maybe used, but the present invention is not limited thereto.

The secondary battery may have various shapes, such as a cylindricalshape, a prismatic shape, or a pouch shape, depending on purposes, andis not limited to a configuration known in the art. The lithiumsecondary battery according to the embodiment of the present inventionmay be a pouch type secondary battery.

Hereinafter, the present invention will be described in more detail,according to examples and experimental examples. However, the presentinvention is not limited thereto.

EXAMPLES Example 1

[Preparation of Electrolyte Solution]

A non-aqueous electrolyte solution was prepared by adding 0.9 mol/L ofLiPF₆, as a lithium salt, based on a total amount of the non-aqueouselectrolyte solution and adding 0.1 mol/L of lithiumbis(fluorosulfonyi)imide and 3 wt % of 2,3-difluorobiphenyi to anon-aqueous organic solvent having a composition in which a volume ratioof ethylene carbonate (EC):ethylmethyl carbonate (EMC) was 3:7.

[Preparation of Lithium Secondary Battery]

A positive electrode mixture slurry was prepared by adding 92 wt % ofLi(Ni_(0.6)Co_(0.2)Mn_(0.2))O_(0.2))O₂ as a positive electrode activematerial, 4 wt % of carbon black as a conductive agent, and 4 wt % ofpolyvinylidene fluoride (PVdF) as a binder to N-methyl-2-pyrrolidone(NMP) as a solvent. An about 20 μm thick aluminum (Al) thin film as apositive electrode collector was coated with the positive electrodemixture slurry and dried, and the coated Al thin film was thenroll-pressed to prepare a positive electrode.

Also, a negative electrode mixture slurry was prepared by adding 96 wt %of carbon powder as a negative electrode active material, 3 wt % of PVdFas a binder, and 1 wt % of carbon black as a conductive agent to NMP asa solvent. A 10 μm thick copper (Cu) thin film as a negative electrodecollector was coated with the negative electrode mixture slurry anddried, and the coated Cu thin film was then roll-pressed to prepare anegative electrode.

A polymer type battery was prepared by a typical method using aseparator formed of three layers ofpolypropylene/polyethylene/polypropylene (PP/PE/PP) with the positiveelectrode and negative electrode thus prepared, and a lithium secondarybattery was then completed by injecting the prepared non-aqueouselectrolyte solution.

Example 2

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 1 except that 0.7 mol/L ofLiPF₆ and 0.3 mol/L of lithium bis(fluorosulfonyl)imide were used.

Example 3

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 1 except that 0.5 mol/L ofLiPF₆ and 0.5 mol/L of lithium bis(fluorosulfonyi)imide were used.

Example 4

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 2 except that 5 wt % of the2,3-difluorobiphenyl was used.

Example 5

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 2 except that 10 wt % of the2,3-difluorobiphenyl was used.

Example 6

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 2 except that 0.5 wt % of the2,3-difluorobiphenyl was used.

Example 7

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 2 except that 2-fluorobiphenylwas used instead of the 2,3-difluorobiphenyl.

Comparative Example 1

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 2 except thatLi(Ni_(0.5)Co_(0.3)Mn_(0.2))O₂ was used as the positive electrode activematerial.

Comparative Example 2

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 2 except thatLi(Ni_(0.8)Co_(0.1)Mn_(0.1))O₂ was used as the positive electrode activematerial.

Comparative Example 3

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 2 except that LiCoO₂ was usedas the positive electrode active material.

Comparative Example 4

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 2 except thatLi(Ni_(0.5)Co_(0.3)Mn_(0.2))O₂ was used as the positive electrode activematerial and 2,3-difluorobiphenyl was not used.

Comparative Example 5

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 2 except that2,3-difluorobiphenyl was not used.

Comparative Example 6

A non-aqueous electrolyte solution and a lithium secondary battery wereprepared in the same manner as in Example 1 except that 0.3 mol/L ofLiPF₆ and 0.7 mol/L of lithium bis(fluorosulfonyl)imide were used.

Experimental Example 1

<Capacity Characteristics After High-Temperature Storage>

The secondary batteries prepared in Examples 1 to 7 and ComparativeExamples 1 to 6 were charged at 1 C to 4.2 V/38 mA under a constantcurrent/constant voltage (CC/CV) condition and then discharged at aconstant current (CC) of 2 C to a voltage of 2.5 V to measure dischargecapacities. Next, after storing the secondary batteries prepared inExamples 1 to 7 and Comparative Examples 1 to 6 at 60° C. for 20 weeks,the secondary batteries were again charged at 1 C to 4.2 V/38 mA under aconstant current/constant voltage (CC/CV) condition at room temperatureand then discharged at a constant current (CC) of 2 C to a voltage of2.5 V to measure discharge capacities. The discharge capacity after 20weeks was calculated as a percentage based on the initial dischargecapacity (discharge capacity after 20 weeks/initial dischargecapacity×100(%)), and the results thereof are presented in Table 1below.

Experimental Example 2

<Output Characteristics After High-Temperature Storage>

After storing the secondary batteries prepared in Examples 1 to 7 andComparative Examples 1 to 6 at 60° C. for 20 weeks, outputs werecalculated from voltage differences which were obtained by charging anddischarging the secondary batteries at 5 C for 10 seconds at roomtemperature. The output after 20 weeks was calculated as a percentagebased on the initial output (output (W) after 20 weeks/initial output(W)×100(%)), and the results thereof are presented in Table 1 below. Theexperiment was performed at a state of charge (SOC) of 50%.

TABLE 1 Positive High-temperature electrode storage character- activeLiPF₆:LiFSI Additive istics (%) material dsd (wt %) Capacity OutputExample 1 NMC622 9:1 DFBP 3 93.9 95.3 Example 2 NMC622 7:3 DFBP 3 95.497.7 Example 3 NMC622 5:5 DFBP 3 92.6 94.1 Example 4 NMC622 7:3 DFBP 592.4 93.3 Example 5 NMC622 7:3 DFBP 10 90.3 91.1 Example 6 NMC622 7:3DFBP 0.5 91.1 92.6 Example 7 NMC622 7:3 FBP 3 89.9 90.4 ComparativeNMC532 7:3 DFBP 3 88.1 90.7 Example 1 Comparative NMC811 7:3 DFBP 3 81.586.3 Example 2 Comparative LiCoO₂ 7:3 DFBP 3 90.7 91.4 Example 3Comparative NMC532 7:3 0 87.7 88.9 Example 4 Comparative NMC622 7:3 090.4 92.1 Example 5 Comparative NMC622 3:7 DFBP 3 87.6 89.9 Example 6

In Table 1, NMC622 represents Li(Ni_(0.6)Co_(0.2)Mn_(0.2))O₂, NMC532represents Li(Ni_(0.5)Co_(0.3)Mn_(0.2))O₂, NMC811 representsLi(Ni_(0.8)Co_(0.1)Mn_(0.1))O₂, DEBT represents 2,3-difluorobiphenyl,and FBP represents 2-fluorobiphenyl.

As confirmed from Table 1, it may be understood that the lithiumsecondary batteries of Examples 1 to 7 exhibited high capacity andoutput even after the high-temperature storage by including thenon-aqueous electrolyte solution which included both of the lithiumbis(fluorosulfonyl)imide and the fluorobiphenyl compound. Among theselithium secondary batteries, the lithium secondary batteries of Examples1 to 5 including 2,3-difluorobiphenyl, as the fluorobiphenyl compound,exhibited better high-temperature storage characteristics than thelithium secondary battery including 2-fluorobiphenyl as thefluorobiphenyl compound. Also, since the lithium secondary batteries ofExamples 1 to 4 included the non-aqueous electrolyte solution includingboth of the lithium bis(fluorosulfonyl)imide and the fluorobiphenylcompound, the lithium secondary batteries of Examples 1 to 4 exhibitedbetter high-temperature storage characteristics than the lithiumsecondary batteries of Comparative Examples 4 and 5 which did notinclude the fluorobiphenyl compound.

When comparing Example 2 with Comparative Examples 1 to 3, Example 2including Li(Ni_(0.6)Co_(0.2)Mn_(0.2))O₂ as the positive electrodeactive material exhibited better high-temperature storagecharacteristics than Comparative Examples 1 to 3 respectively includingLi(Ni_(0.6)Co_(0.3)Mn_(0.2))O₂, Li(Ni_(0.8)Co_(0.1)Mn_(0.1))O₂, andLiCoO₂ as the positive electrode active material.

Also, when comparing Examples 1 to 3 with Comparative Example 6, in acase in which lithium-manganese-cobalt-based oxide,Li(Ni_(0.6)Co_(0.2)Mn_(0.2))O₂, was included as the positive electrodeactive material, it may be confirmed that the high-temperature storagecharacteristics of the lithium secondary batteries were degraded whenthe amount of LiFSI was increased in comparison to that of LiPF₆.

Experimental Example 3

<Overcharge Evaluation>

The lithium secondary batteries prepared in Example 1 and ComparativeExample 5 were overcharged to 8.3 V under a constant current/constantvoltage (CC/CV) condition of 1 C (775 mAh)/12 V from a charged state at25° C., changes in temperature and voltage of the battery at that timewere measured, and the results thereof are presented in FIG. 1.

Referring to FIG. 1, it may be confirmed that the lithium secondarybattery of Example 1, which included the non-aqueous electrolytesolution including the 2,3-difluorobiphenyl additive had a lower SOC atthe time of short circuit caused by overcharge and a lower temperatureat the center of the battery than the lithium secondary battery ofComparative Example 5 which did not include the 2,3-difluorobiphenyladditive.

1. A lithium secondary battery comprising: a non-aqueous electrolytesolution including lithium bis(fluorosulfonyl)imide (LiFSI) and afluorobiphenyl compound; a positive electrode including alithium-nickel-manganese-cobalt-based oxide as a positive electrodeactive material; a negative electrode; and a separator.
 2. The lithiumsecondary battery of claim 1, wherein the non-aqueous electrolytesolution further comprises a lithium salt excluding the lithiumbis(fluorosulfonyl)imide.
 3. The lithium secondary battery of claim 2,wherein a mixing ratio of the lithium salt and the lithiumbis(fluorosulfonyl)imide is in a range of 1:0.01 to 1:1 as a molarratio.
 4. The lithium secondary battery of claim 1, wherein the lithiumbis(fluorosulfonyl)imide has a concentration of 0.01 mol/L to 2 mol/L inthe non-aqueous electrolyte solution.
 5. The lithium secondary batteryof claim 2, wherein the lithium salt comprises one selected from thegroup consisting of LiPF₆, LiAsF₆, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiBF₆,LiSbF₆, LiN(C₂F₅SO₂)₂, LiAlO₄, LiAlCl₄, LiSO₃CF₃, and LiClO₄, or amixture of two or more thereof.
 6. The lithium secondary battery ofclaim 1, wherein the lithium-nickel-manganese-cobalt-based oxidecomprises an oxide represented by Formula 1:Li_(1+x)(Ni_(a)Co_(b)Mn_(c))O₂   [Formula 1] wherein, in Formula 1,0.55≤a≤0.65, 0.18≤b≤0.22, 0.18≤c≤0.22, −0.2≤x≤0.2, and x+a+b+c=1.
 7. Thelithium secondary battery of claim 1, wherein the non-aqueous organicsolvent comprises a nitrile-based solvent, linear carbonate, cycliccarbonate, ester, ether, ketone, or a combination thereof.
 8. Thelithium secondary battery of claim 7, wherein the cyclic carbonatecomprises one selected from the group consisting of ethylene carbonate(EC), propylene carbonate (PC), and butylene carbonate (BC), or amixture of two or more thereof, and the linear carbonate comprises oneselected from the group consisting of dimethyl carbonate (DMC), diethylcarbonate (DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC),methylpropyl carbonate (MPC), and ethylpropyl carbonate (EPC), or amixture of two or more thereof.
 9. The lithium secondary battery ofclaim 7, wherein the nitrile-based solvent comprises at least oneselected from the group consisting of acetonitrile, propionitrile,butyronitrile, valeronitrile, caprylonitrile, heptanenitrile,cyclopentane carbonitrile, cyclohexane carbonitrile,2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile,trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile,and 4-fluorophenylacetonitrile.
 10. The lithium secondary battery ofclaim 1, wherein the fluorobiphenyl compound is a compound representedby Formula 2:

wherein, in Formula 2, n is an integer of 1 to
 5. 11. The lithiumsecondary battery of claim 1, wherein the fluorobiphenyl compound is2,3-difluorobiphenyl.
 12. The lithium secondary battery of claim 1,wherein an amount of the fluorobiphenyl compound is in a range of 0.5 wt% to 10 wt % based on a total weight of the non-aqueous electrolytesolution.
 13. The lithium secondary battery of claim 1, wherein thelithium bis(fluorosulfonyl)imide and the fluorobiphenyl compound areincluded in a weight ratio of 1:0.02 to 1:10.
 14. A lithium secondarybattery comprising the non-aqueous electrolyte solution of claim 1.